Post-tensioning is a type of steel reinforcement for concrete structures. It typically refers to the method of placing wet concrete around unstressed tendons that have been coated with grease and encased in a plastic sheath. After sufficient hardening of the concrete the tendons are pulled, or post-tensioned, to provide strength to the structure. The tendons are then anchored and cut off just inside the face of the structure in what are termed grout pockets. The grout pockets surrounding these tendon ends are then filled with a concrete grout.
The basic procedures used in prestressing or post-tensioning concrete structures have been known since the 1940's, and development of those procedures is traceable to the late 1920's. However, general acceptance and extensive use of post-tensioning did not begin until the late 1960's or early 1970's. The use of post-tensioning is especially suited to slab construction such as multi-level floor slabs in parking garages and buildings, or slabs-on-ground such as in residential construction. The post-tensioned slab-on-ground foundation for residential construction has become the system of choice for many builders in areas that have a relatively flat terrain. The primary reason for this is its economy and ease of installation.
Corrosion protection is provided for most of the tendon length by a coating of grease and a plastic sheath installed by the manufacturer. However, the plastic sheath must be removed at each terminal end for anchorage. This results in a length of bare tendon and an anchor assembly that are protected only by the concrete grout installed by the tensioner at the time of stressing. Corrosion problems typically result due to inadequate concrete covering the terminal end of the tendon. The end of the tendon must be cut off a sufficient distance inside the face of the slab to prevent corrosion from starting and cracking the concrete grout seal. The required clearance distance for reinforcing steel from an exterior formed concrete surface is typically 2-inches. This distance is necessary to prevent contact of the steel with weathered, or carbonated, concrete. Carbonated concrete is concrete that has reacted with CO.sub.2 in the atmosphere over a period of time. This carbonation lowers the pH of the concrete to a level that is no longer non-corrosive to steel, and the steel begins to corrode. This is a condition that is beginning to affect numerous concrete structures where steel is present within the zone of carbonation. This condition is especially serious in post-tensioned structures since most pocket formers for post-tensioning anchorages make a pocket only 1.5-inches deep, and only recently has a tool become available for cutting the tendon off more than about 1/4-inch inside the pocket.
Although designed to be maintenance free for decades, it became apparent in the early 1980's that structures installed in corrosive environments were failing after only a few years due to failure of the grout seal and severe corrosion at the terminal end anchorages. Subsequently, coated tendons and encapsulating anchors were developed and are now used for corrosive environments. However, residential construction has continued with very little change from the methods used in the 1960's and 1970's. It has now become apparent that premature failure of concrete grout seals is beginning to cause failure of terminal end anchorages in all types of structures, and specifically in large numbers of single and multi-family residential structures. Most owners of post-tensioned structures are unaware of the type of reinforcing they have, and are not knowledgeable about inspection or maintenance of the terminal ends of the post-tensioned tendons.
The present invention is directed at the large numbers of tendons in new and existing service that are in need of additional protection to prevent corrosion and eventual failure.